Aldehyde functional siloxanes are of great interest as commercial intermediates in the silicone industry due to their reactivity and hence versatility to form various organofunctional siloxanes. However, limited routes to aldehyde functional silicones exist and very few, if any, are presently practiced via large scale industrial processes.
One means for preparing aldehyde functional siloxanes involves the reaction of a hydride functional siloxane with an acetal containing an alpha-olefin group, for example J. Org. Chem., (1970) 35(12) 4180. After the hydrosilylation is complete, the aldehyde is then liberated using an acidic catalyst and water. This reaction is typically biphasic, generates significant amounts of waste by-products and requires multiple steps to recover the desired product.
Ozonolysis is another route to aldehyde functional siloxanes. This route involves the exposure of an olefin functional siloxane to ozone to form silicone ozonides. The ozonides can be further reacted under acidic conditions to form aldehydes, see for example U.S. Pat. No. 5,739,246.
The Grignard coupling of bromophenyl dioxolanes with chlorosilanes and subsequent hydrosilylation with vinyl functional siloxanes, as described in the Journal of the Chemical Society, Dalton Translation (2001), (7), 1123-1127, is another method for preparing aldehyde functional siloxanes.
Another possible route to aldehyde functional silicones is the hydrosilylation of aldehydes that also contain other unsaturated carbon-carbon bonds with SiH functional siloxanes. This route would be desirable because of the versatililty of hydrosilylation reactions. However, this route has presumably not been employed due to the competing reaction of the addition of the SiH across the aldehyde carbonyl group resulting in a high percentage of hydrolyzable SiOC bonds.
EP0231093 B1 teaches a method of making aldehyde functional silicone gels by the hydrolysis of acetal and aldehyde functional silanes.
The present inventors have unexpectedly discovered a process to successfully hydrosilylate unsaturated groups on aldehydes, while minimizing the formation of SiOC bonds.